Proximity switches are known in the art. Such known proximity switches utilize an oscillator drive circuit in combination with an induction tank circuit. The tank circuit includes an induction coil as a means for sensing the presence of an object such as metal. The induction coil is constructed such that it generates a magnetic field in an area surrounding the coil. The magnetic field induces eddy currents in a conductive object which comes within the generated magnetic field. Such objects are known in the art as targets. Once a target comes within the magnetic field of the coil, energy is drawn from the induction coil. A typical induction proximity switch selects components of the oscillator and tank circuit to insure that oscillations occur when a target is absent from the magnetic field of the induction coil. When a target comes within the magnetic field of the induction coil, the oscillation amplitude is attenuated due to the loss of energy caused by the induction coil magnetic field inducing eddy currents in the target. The amount of the oscillation attenuation is directly related to the distance between the target and the induction coil.
A predetermined distance between the induction coil and the target is selected as the point where the output of the proximity switch changes an electrical state to indicate the presence of a target. This distance is known as the switch trip-point. A circuit within the proximity switch monitors the oscillation amplitude and generates a signal at the output of the proximity switch indicative of the fact that the target has come within the trip point distance.
One problem with prior art proximity switches occurs when power is first applied to the proximity switch or power is switched from a power-off to a power-on condition. If a target is located within the magnetic field of the induction coil but beyond the trip-point distance during this power-up condition, a false indication of target presense occurs. When power is applied to the proximity switch and the target is within the field, the oscillations of the tank circuit build up slowly due to the additional dampening of the induction coil caused by an energy transfer between the coil and the target. Thus, the oscillation amplitude will not reach a proper level within a given amount of time and a false indication that a target is within the trip-point distance occurs for the period of time required for the oscillator amplitude to reach this proper level.
One solution to this problem has been to provide a time delay circuit to disable the proximity switch output signal until after a certain amount of time has lapsed from an initial power-on condition. This solution has not proved satisfactory since the amount of time delay needed to insure that a false signal will not occur adversely affects the activation time of a switch after a power-up condition occurs.
Another problem with prior art proximity switches lies in the design of the detector circuit that is used to monitor the amplitude of the oscillations. The detector usually draws an amount of current from the oscillator circuit which changes as the target position changes. Consequently, hysteresis in the level of oscillation will be introduced or modified by the presence of the detector circuit. The magnitude of the hysteresis depends upon component parameters of the detector circuit which are subject to great variance.
Still another problem with prior art proximity switches lies in the design of the comparator circuit that is used to determine when the target has come within the trip point distance. The comparator circuit compares a D.C. output signal from the detector circuit against a reference voltage. The level of the D.C. voltage from the detector circuit is a function of the target distance from the induction coil. Noise or ripple on the detector output signal of the detector circuit can cause the comparator to chatter and result in a false target present output signal to occur at the output of the proximity switch.